1. Field of the Invention
The present invention relates to a tracking control apparatus for an optical disk, such as an optical disk of pit-formation type, a magneto-optical disk, etc. More particularly, the invention relates to a tracking control apparatus capable of performing an accurate tracking action with no use of extra position sensor or switch and without a high accuracy of finishing or processing.
2. Description of the Related Art
In an optical disk, an optical head with an objective lens mounted thereon is arranged to be movable in a radial direction of the disk to make an access to an arbitrary track on the disk.
K. Bulthuis, M. G. Carasso, J. P. J. Heemskerk, P. J. Kivits, W. J. Kleuters, and P. Zalm, "Ten billion bits on a disk," IEEE spectrum, pages 28 through 33, August 1979 describes tracking servo control of the conventional optical disk of pit-formation type. K. Torazawa, S. Sumi, S. Murata, S. Minechika, and Y. Ishii, "Erasable digital audio disk system", Applied Optics, Vol. 25, No. 22, pages 3995, 15 Nov. 1986 describes tracking servo with push-pull method of the conventional magneto-optical disk.
Such conventional optical disk apparatus executes an initial action in the following four steps.
In the first step, the objective lens is vertically moved to the disk to position an optical spot converged at the objective's focal point within a vertical range, where a focusing servo control may be implemented near a recording layer of the disk.
In the second step, the focusing servo is activated.
In the third step, a tracking servo is activated to make the optical spot follow a track on the disk.
In the fourth step, address information is read from the track on which the optical spot is centered.
In the conventional optical disk apparatus, the optical head is moved in the radial direction of the disk within a head movable range in which the head can move radially, to make an access to an arbitrary track in the tracking enable region on the disk.
The head movable range must be designed such that the optical head can move to all tracks even with the narrowest possible range which could occur by errors upon processing and assembling.
Actually, the head movable range is widely set taking into consideration an eccentricity of the disk itself, vibrations and positional displacement of the spindle motor, and other factors.
Supposing the head movable range, which is preliminarily widely set for the above reason, has become for example the widest due to the errors upon processing and assembling of apparatus, and if the optical head is positioned at an edge of the head movable range, there is a possibility that the tracking enable region is absent on the optical axis of the objective lens. The tracking enable region is the region where guide grooves, servo marks or the likes are formed.
Then, before servo tracking or pull-in action, the optical head is required to move in the radial direction of the disk in the head movable range to be located in the tracking enable region.
In order to judge whether the optical head is present in the tracking enable region or not, a sensor or switch has been disposed to detect a position of optical head in the radial direction of the disk, raising a cost.
If the processing errors and the disk eccentricity are minimized in order to omit such a sensor or switch, the optical head would be always present within the tracking enable region wherever the optical head is located in the radial direction of the disk.
However, this causes another problem to raise a processing cost and a production cost of disk.